


Principles of Reasoning

by longleggedgit



Category: Tennis no Oujisama | Prince of Tennis
Genre: M/M
Language: English
Status: Completed
Published: 2014-01-19
Updated: 2014-01-19
Packaged: 2018-01-09 06:28:43
Rating: General Audiences
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 3,060
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/1142607
Author URL: https://archiveofourown.org/users/longleggedgit/pseuds/longleggedgit
Summary: <blockquote class="userstuff">
              <p>Kaidoh is having trouble understanding logic. Inui is an eager tutor.</p>
            </blockquote>





	Principles of Reasoning

**Author's Note:**

> Written in 2007 (my first ever InuKai). Thanks to [](http://anjenue.livejournal.com/profile)[](http://anjenue.livejournal.com/)**anjenue** for the beta. ♥

It’s nothing out of the ordinary for Momoshiro and Kaidoh to come into the locker room in the midst of a quarrel, but Inui nevertheless perks up, alert and prepared to take a detailed mental note. Some of his best data about both Momoshiro and Kaidoh comes from listening in on their arguments, and today’s sounds particularly vehement.

“Hey!” Momoshiro yells when Kaidoh punches him in the shoulder. “Don’t take it out on me just because you’re failing math!”

“I’m not _failing_ ,” Kaidoh growls, and Inui raises an eyebrow at the dangerous edge to his voice.

“Fifty-four percent is failing in _my_ math class,” Momoshiro says, and this time when Kaidoh punches him he goes straight for the face.

The two are in a tangled mess on the floor, with Inui seriously considering intervening even despite the seventy-four percent chance of sustaining personal bodily harm, when Tezuka opens the door. Kaidoh sees him first and freezes mid-punch, his arm suspended above Momoshiro’s head. Momoshiro gets in one more sharp kick before he realizes something is amiss and freezes too, not daring to look behind him but most likely all too aware that there’s only one person in the world whose presence could make Kaidoh forget about a fight.

“Momoshiro! Kaidoh! Thirty laps!” Tezuka says, and Momoshiro and Kaidoh leap to their feet and wrestle out the door without a word between them. Inui adjusts his glasses.

“What was that all about?” Eiji asks when they’re gone.

“Math,” Inui says, and he reaches into his bag and takes out his notebook.

~

Once practice is over, Inui waits a full twelve minutes longer than he typically lingers in the locker room before Kaidoh emerges from the showers. Kaidoh is often the last to finish showering. Inui speculates it has something to do with his hair, although this hypothesis is a weak one, as Fuji is consistently one of the first to leave the showers after practice. Then again, Fuji tends to defy most data.

“Senpai?” Kaidoh says, a towel around his waist and in the midst of drying his hair with another.

“Kaidoh,” Inui says. “Is it true you’re struggling with your math class?”

Kaidoh’s cheeks flare a brilliant red, and he turns to face his locker in an effort to hide it. “It’s not the math,” he says stiffly. “I’m good at math.”

Inui says nothing, confident that Kaidoh will elaborate if he stares at him for longer than seven seconds.

Eight seconds later, Kaidoh hisses. “We’re doing a unit on logic,” he says. “I didn’t do very well on the last test.”

“You’re having difficulties understanding logic?” Inui asks calmly.

“There are no numbers!” Kaidoh opens his locker door, yanks out his clothes, and slams it shut. “We shouldn’t be learning about it in math, anyway!”

Inui tries to make his smile appear friendly, but from the nervous look Kaidoh gives him, he surmises the attempt has failed. He makes a note to work on friendly.

“I am quite adept at math, and completed the same course you are currently taking last year,” Inui reminds him.

Kaidoh looks uncomfortable, and he pointedly avoids Inui’s gaze, fiddling with the clothes slung over his arm.

“I would be happy to tutor you,” Inui says.

“I – I wouldn’t want to trouble you, Senpai.”

“It’s no trouble.” Inui clasps his hands in his lap and smiles more broadly. “When is your next test?”

Kaidoh looks at him sideways, apparently waging some sort of internal struggle, before he answers, “Friday.”

Inui is silent for a moment. “As in, tomorrow?”

Kaidoh closes his eyes. “Yes,” he says through gritted teeth.

“Ah.”

“I’m sure I’ll be –“

“Would you like to come over to my house to study, Kaidoh?”

Kaidoh’s clothes slip off of his arm and he bends down to pick them up, the expression on his face one akin to terror. Inui makes a note that he really, really needs to work on friendly.

“I’m sure it’s an inconvenience to you, Senpai,” Kaidoh stammers.

“Not at all.” Inui bends down to pick up Kaidoh’s boxers, which he missed, and Kaidoh’s flush deepens to a shade of red that borders on unhealthy. “I’m happy to be of assistance.”

“I’ll – go change,” Kaidoh says. He takes two steps toward the shower room, freezes, and then returns to grab the boxers Inui still holds in his outstretched hand.

“I’ll wait outside,” Inui says, and he just manages not to laugh.

~

In two years of observing Kaidoh, Inui has never seen him quite so on edge. He’s uncertain if this is because of the impending test, the sudden invitation to Inui’s house, or the state of Inui’s bedroom, but his best guess is it’s some combination of the three.

“I apologize for the mess,” Inui says, and while Kaidoh shakes his head and mutters something about it being fine, his eyes still drift from the sports magazines strewn across the floor to the scrawling notes that cover the walls. Inui finds himself feeling unexpectedly self-conscious, and for the first time in months considers the possibility of cleaning his room.

“Well.” Inui claps his hands together loudly, and Kaidoh jumps. “Let’s get started, shall we?”

“Y-yes,” Kaidoh agrees. Inui pats the side of the bed, and Kaidoh slowly takes a seat beside him, eyes fixed on the floor.

“What chapter are you having trouble with?” Inui asks.

By Kaidoh’s hesitation, Inui guesses the answer is most of them. “The test will cover mostly three and four,” Kaidoh says after a beat. He bends down to take the textbook out of his bag and hands it to Inui.

“Ah!” Inui brightens the second the book is open. “You’re analyzing the validity and invalidity of arguments?”

“And identifying fallacies,” Kaidoh says. He appears less on edge now that they have a set task to accomplish, to Inui’s relief, and Inui spreads the book out between them so they can look on at the same time.

“Let’s start here,” Inui says, pointing to the first exercise. “’Titanium combines readily with oxygen, nitrogen, and hydrogen, all of which have an adverse effect on its mechanical properties. As a result, titanium must be –‘“ he cuts himself short when he catches the look on Kaidoh’s face, which is straddling a fine line between perplexed and murderous.

“Kaidoh?”

“Could we try a different example first?” Kaidoh is already flushed, but he manages to keep from hissing.

“Of course.” Inui scans the page briefly before landing on another candidate. “Here’s one: ‘Since the good, according to Plato, is that which furthers a person’s real interests, it follows that in any given case when the good is known, men will seek it.’ Here, we simply need to identify the premises before. . . .” Inui trails off again, looking down at the pencil in Kaidoh’s hand, which has just snapped into two pieces.

“Sorry,” Kaidoh says quickly, and he slides off the bed to search for a garbage bin.

“It would appear that the wording of these examples is what’s causing you trouble, Kaidoh,” Inui says. “And you can just leave those anywhere on the floor.”

Kaidoh looks absolutely horrified by the suggestion, but he says nothing, merely slips the pencil pieces into a side pocket on his book bag and returns to his seat on the bed.

Inui clears his throat. “Let’s try putting some examples into terms that are easier to understand. I’ll give you an example I make up, and you tell me what the premises are and what the conclusion is.”

“Okay,” Kaidoh says resignedly.

“Example one,” Inui says. “All tennis players are athletes.” Kaidoh instantly lifts his head. “All athletes exercise for six hours per day. Kaidoh is a tennis player, therefore Kaidoh exercises for six hours per day.”

Kaidoh is observing Inui with wonder and possibly suspicion. “Are you allowed to do that?”

Inui grins. “Of course. It’s all the same concept. Now, the premises . . . ?”

It only takes Kaidoh a few seconds to answer this time. “All tennis players are athletes, and all athletes exercise for six hours per day.” He is almost smiling.

“And the conclusion?”

Kaidoh’s blush spreads to the tips of his ears, but his reply is confident. “Kaidoh exercises for six hours per day.”

“Very good.” Inui positively beams at him, and after a moment, Kaidoh smiles back. “Now, can you tell me if it is valid or invalid?”

Kaidoh frowns again. “I don’t exercise six hours every day,” he says. “Even with your training schedule –“

Inui holds up a finger and Kaidoh goes silent. “Remember, Kaidoh, the _validity_ of an argument has nothing to do with its _truth_. Validity only suggests that the premises offer adequate support to draw the conclusion.”

“Oh.” Kaidoh continues to frown. “Then – it’s valid?”

“Yes,” Inui says, and Kaidoh’s frown melts away. “Now, if you want to analyze the truth value of the argument, it would be valid but unsound, because you in fact do _not_ exercise for six hours every day.”

“That make sense,” Kaidoh says.

“I’m glad.” Inui flips to the next page in the textbook and bends over. Kaidoh bends over as well, to the point that their foreheads are almost touching.

Really, it’s not Inui’s fault that he starts getting ideas.

“Shall we try another one?” Inui asks, and Kaidoh nods eagerly. “This time, just tell me the validity, and whether it is sound or unsound,” he says.

Kaidoh nods again. “Okay.”

“It is the case that all doubles partners are dating,” Inui says. Kaidoh’s eyebrows knit together in confusion, but he does not interject. “Eiji and Oishi are doubles partners, so Eiji and Oishi are dating.”

“Senpai?”

Inui pushes his glasses up the bridge of his nose. “Are you having trouble?”

“No, er –“ Kaidoh pauses and bites his lower lip. “Valid,” he says slowly. “And . . . unsound.”

“You think so?” Inui says, surprised. “Well, there is of course room for interpretation where the truth value is concerned with certain questions . . . very good, let’s try another.”

Kaidoh looks as if he’s about to make an objection, then closes his mouth and crosses his arms in front of his chest, eyes wide and disbelieving.

“We’ll try one with an unquestionable truth value,” Inui suggests, and Kaidoh shifts in his seat.

Inui calculates he will lower his chances of eliciting a violent response to the next question if he pretends to consider it, so he does, tilting his chin in exaggerated thoughtfulness. “It is the case that all doubles partners are dating,” he says at last. “Momoshiro and Kaidoh are doubles partners, so –“

“Invalid and unsound,” Kaidoh practically yells, and Inui has to duck his head to hide his amusement.

“Remember, the truth value has nothing to do with validity,” Inui reminds him.

Kaidoh just glares, refusing to amend his answer. “By that logic, we’d be dating, too,” he says.

“Hmm.”

Inui waits just long enough for Kaidoh’s blush to reach his collarbones before he breaks the silence. “Let’s try another, shall we?”

“Good idea,” Kaidoh grumbles.

Before he speaks again, however, Inui picks up his notebook and scribbles something, snapping it shut when Kaidoh tries to glance over his shoulder.

“If Inui removes Kaidoh’s bandana,” Inui says, “Kaidoh will hit Inui in the face.”

Kaidoh’s glare only intensifies, which comes as no surprise to Inui, who calculated a mere fourteen percent chance Kaidoh wouldn’t get angry at the very thought of having his bandana removed without consent. Inui opens his mouth to finish with a conclusion, but Kaidoh again cuts him short.

“Valid,” Kaidoh says huffily, “and sound.”

Inui is quiet for a moment. “Oh,” he says, and he picks up his notebook and takes another note.

“What are you writing?” Kaidoh asks, suspicious.

“Research,” Inui says with a vague wave of the hand, and he again snaps the notebook shut and drops it on the floor. “Just a few more,” he says. “I think you’re getting the hang of it now.”

Kaidoh’s expression softens and he leans back. “Okay,” he says.

Inui smiles, trying not to betray the nervous knot twisting in his gut. This is easily the most dangerous experiment he has ever attempted.

“This time,” Inui says, “you tell me the logical conclusion to follow the premises.” Kaidoh nods.

“Tennis players have strong leg muscles,” Inui says. Kaidoh looks greatly relieved to have left the realm of specific examples. “Persons with strong leg muscles are attractive.”

Kaidoh’s look of relief vanishes instantly.

“Kaidoh is a tennis player. Therefore?”

Inui has judged that, by this point, it would be physically impossible for Kaidoh to get any redder, but as Kaidoh begins to fidget Inui squints his eyes, wondering if he might have made an error in his calculations.

“I have to go to the bathroom,” Kaidoh says suddenly, and he jumps off the bed and dashes out the door, preventing any further scrutiny on Inui’s part.

Inui thinks he’s probably pushing the testing conditions a little too far, but if there is one thing he can’t stand, it’s leaving an experiment unfinished. He closes Kaidoh’s textbook gently, lowers it to the floor, and works on his friendly smile until the door opens again and Kaidoh reappears, face considerably less flushed and shining with dampness.

“The correct answer was, ‘Kaidoh is attractive,’” Inui says before he can stop himself.

Kaidoh closes his eyes and stuffs his hands in his pockets. “Oh.”

A tense silence falls over them, and Inui feels his palms start to sweat, wonders if he’s already botched this one, if he’s going to have to start over from square one. He’s come _too far_ to go back to square one.

“Are you up for one more?” Inui hazards.

And despite what Inui can only estimate to be overwhelming odds, Kaidoh says, “Sure.”

Inui waits until Kaidoh has returned to the bed, seated on the edge stiffly with his hands fisted in Inui’s comforter. Inui begins to worry his calculations are losing their accuracy, because he is almost certain that this time, Kaidoh sits more than three centimeters closer.

“All you have to do is tell me the validity and if it’s sound or unsound,” Inui says. He notices he is leaning in toward Kaidoh but can’t be bothered to stop.

“O-okay,” Kaidoh stammers. His eyes are fixed steadfastly on the opposite wall.

Inui stops leaning in, but only because there is scarcely space left between them. When he speaks, his breath skirts across Kaidoh’s cheek and causes him to shiver.

“If Inui kisses Kaidoh,” Inui says, and Kaidoh shivers again, “Kaidoh will hit Inui in the face.”

Kaidoh’s shoulders tense up, his breath stops, and his eyes clench tightly closed. Inui just has time to think this is definitely, without a doubt the most dangerous experiment he has ever attempted before he presses his lips to Kaidoh’s cheek.

He finds himself breathing out a sigh of relief even before he has removed his lips, before he has any indication what the results of this experiment might be. It’s always better at this point, waiting for a hypothesis to either be supported or proven false, knowing the hard part is over and the results are out of his hands.

Then Kaidoh lets out a quiet, “Valid,” and Inui realizes the lurch in his chest might indicate the first time he has ever been so afraid of failure.

“Kaidoh. . . .” he says shakily.

Kaidoh turns his head before Inui can pull away, and when their noses brush together he stops, resting there, eyes still closed and lips parted slightly.

“Unsound,” he says.

Inui’s sigh is audible this time, and he can’t prevent himself from breaking into a grin. “Good,” he says, and Kaidoh opens his eyes just as Inui takes his face in both hands, closing them again when their lips press together.

There are preparations Inui made for this moment, research and practice and observation and extensive note-taking, but he forgets it all and finds he doesn’t care, not when their lips fit so smoothly together anyway, not when Kaidoh opens his mouth without even being coaxed, just enough to allow Inui’s tongue to swipe across his teeth before sucking in his bottom lip. Kaidoh makes a soft noise somewhere between a moan and a whimper, and his hands are on Inui’s chest, clutching at his shirt collar, pulling him closer. Inui keeps one hand on Kaidoh’s face, stroking his cheek with his thumb, and he slides the other hand to the back of his head, tangling in his hair and tugging so Kaidoh will tilt back and he can deepen the kiss.

Kaidoh breaks away then, which Inui would have predicted if he’d been capable of focusing on anything but Kaidoh’s hands on his chest, the taste of his mouth, the pattern of their mingled breathing. Inui can’t even really be disappointed, because he’s still glowing from the success of his experiment – and besides that, Kaidoh hasn’t made some excuse and run out the door yet. In fact, his fingers are still toying with Inui’s collar, and after a moment he leans in to press his forehead against Inui’s.

“You got one incorrect,” Inui says, and Kaidoh looks up sharply, cheeks redder than before he escaped to the bathroom.

“Senpai?”

Inui smiles and lowers his hand, which is clutching Kaidoh’s crumpled bandana. Kaidoh blinks, runs a hand through his mussed hair, and grabs the bandana back.

“I’ll hit you later,” he says, and as he pulls Inui in for another kiss, Inui thinks the black eye will be very, very worth it.

~

“I can’t believe it,” Momoshiro groans, and he throws his crumpled exam against the lockers petulantly. “The Viper actually did better than me.”

Inui tries unsuccessfully to hide his pleasure at hearing this. “Oh?” He turns to watch Kaidoh, who is silently removing his exercise clothes from his locker. “How did you do on your test, Kaidoh?”

Kaidoh shrugs. “Eighty-two percent,” he says, and Inui frowns.

“Really? I would’ve thought –“

“We never got to fallacies on Thursday,” Kaidoh says, very carefully avoiding Inui’s eyes.

Inui coughs to cover up his grin. “I see,” he says, adjusting his glasses. “Perhaps you’d like to come over tonight so we can cover them for your next test?”

“Okay,” Kaidoh says, a little too quickly.

“Can I come, too?” Momo asks. “I did terrible.”

“No,” Inui and Kaidoh say at once.

Kaidoh blushes.

_end_  



End file.
